function fearNotLetter(str) {
    for (let i = 1, len = str.length; i <= len; i++) {
        const currentCode = str.charCodeAt(i);
        const prevCode = str.charCodeAt(i - 1);
        if (~~(currentCode - prevCode) > 1)
            return String.fromCharCode(prevCode + 1);
    }
}

fearNotLetter("abce");